1. Field of the Invention
The present invention relates generally to methods and apparatus for making plastic lenses using ultraviolet light.
2. Description of Related Art
It is conventional in the art to produce optical lenses by thermal curing techniques from the polymer of diethylene glycol bis(allyl)-carbonate (DEG-BAC). In addition, optical lenses may also be made using ultraviolet ("UV") light curing techniques. See, for example, U.S. Pat. No. 4,728,469 to Lipscomb et al., U.S. Pat. No. 4,879,318 to Lipscomb et al., U.S. Pat. No. 5,364,256 to Lipscomb et al., and U.S. Pat. No. 5,415,816 to Buazza et al., U.S. patent application Ser. No. 07/021,913 filed Mar. 4, 1987, Ser. No. 07/425,371 filed Oct. 26, 1989, Ser. No. 07/931,946 filed Aug. 18, 1992, Ser. No. 08/123,886 filed Sept. 20, 1993, Ser. No. 07/932,812 filed Aug. 18, 1992, all of which are hereby specifically incorporated hereby by reference.
Curing of a lens by ultraviolet light tends to present certain problems that must be overcome to produce a viable lens. Such problems include yellowing of the lens, cracking of the lens or mold, optical distortions in the lens, and premature release of the lens from the mold. In addition, many of the usefull UV-curable lens forming compositions exhibit certain characteristics which increase the difficulty of a lens curing process. For example, due to the relatively rapid nature of ultraviolet light initiated reactions, it is a challenge to provide a composition which is UV curable to form an eyeglass lens. Excessive exothermic heat tends to cause defects in the cured lens. To avoid such defects, the level of photoinitiator may be reduced to levels below what is customarily employed in the ultraviolet curing art.
While reducing the level of photoinitiator addresses some problems, it may also cause others. For instance, lowered levels of photoinitiator may cause the material in regions near an edge of the lens and proximate a gasket wall in a mold cavity to incompletely cure due to the presence of oxygen in these regions (oxygen is believed to inhibit curing of many lens forming compositions or materials). Uncured lens forming composition tends to result in lenses with "wet" edges covered by sticky uncured lens forming composition. Furthermore, uncured lens forming composition may migrate to and contaminate the optical surfaces of the lens upon demolding. The contaminated lens is then often unusable.
Uncured lens forming composition has been addressed by a variety of methods (see, e.g., the methods described in U.S. patent application Ser. No. 07/931,946). Such methods may include removing the gasket and applying either an oxygen barrier or a photoinitiator enriched liquid to the exposed edge of the lens, and then re-irradiating the lens with a dosage of ultraviolet light sufficient to completely dry the edge of the lens prior to demolding. During such irradiation, however, higher than desirable levels of irradiation, or longer than desirable periods of irradiation, may be required. The additional ultraviolet irradiation may in some circumstances cause defects such as yellowing in the lens.
The low photoinitiator levels utilized in many ultraviolet curable lens forming compositions may produce a lens which, while fully-cured as measured by percentage of remaining double bonds, may not possess sufficient crosslink density on the lens surface to provide desirable dye absorption characteristics during the tinting process.
Various methods of increasing the surface density of such UV curable lenses are described in U.S. patent application Ser. No. 07/931,946. In one method, the lens is demolded and then the surfaces of the lens are exposed directly to ultraviolet light The relatively short wavelengths (around 254 nm) provided by some UV sources (e.g., a mercury vapor lamp) tend to cause the material to crosslink quite rapidly. An undesirable effect of this method, however, is that the lens tends to yellow as a result of such exposure.
Another method involves exposing the lens to relatively short wavelengths while it is still within a mold cavity formed between glass molds. The glass molds tend to absorb the more effective short wavelengths, while transmitting wavelengths of about 365 nm. This method generally requires long exposure times and often the infrared radiation absorbed by the lens mold assembly will cause premature release of the lens from a mold member. The lens mold assembly may be heated prior to exposure to high intensity ultraviolet light, thereby reducing the amount of radiation necessary to attain a desired level of crosslink density. This method, however, is also associated with a higher rate of premature release.
It is well known in the art that a lens mold/gasket assembly may be heated to cure the lens forming composition from a liquid monomer to a solid polymer. It is also well known that such a lens may be thermally postcured by applying convective heat to the lens after the molds and gaskets have been removed from the lens.
In this application the terms "lens forming material" and "lens forming compositions" are used interchangeably.